A fuel cell is an electrochemical cell that converts chemical energy directly into electrical energy through a chemical reaction between an electrolyte, a fuel, and an oxidizer. Fuel cells thus differ from batteries in that they consume reactants, which must be replenished, while batteries merely store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable. The use of fuel cells is becoming of ever-greater interest as a potentially cleaner and more efficient manner of producing electrical energy relative to combustion-based power generation processes and systems. Two forms of fuel cells presently predominate development activities: Proton Exchange Membrane Fuel Cells (PEMFC) and Solid Oxide Fuel Cells (SOFCs).
Proton Exchange Membrane Fuel Cells (also known as Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are discussed in US20060090317; US20040096718; WO03052847; and WO04024796. PEMFCs involve the use of a membrane to transform the chemical energy liberated during the electrochemical reaction of reactants (such as hydrogen and oxygen) to electrical rather than thermal energy. Technical constraints have limited the applicability of PEMFCs. The membrane must be capable of conducting hydrogen ions while being impermeable to electrons and gases. It must also be resistant to both the reducing environment at the cathode and the oxidative environment at the anode. The most commonly used membrane is Nafion® (DuPont), which relies on liquid water humidification of the membrane to transport protons. PEMFCs do not operate at temperatures above 80-90° C., and their efficiencies are presently only in the range of 40-60% Higher Heating Value of Hydrogen (HHV; defined as the amount of heat released by the combustion of a specified quantity of material (measured at initial and final temperatures of 25° C.)). Additionally, PEMFCs require expensive catalysts (such as platinum).
Solid Oxide Fuel Cells (SOFCs) are discussed in U.S. Pat. Nos. 7,211,236; 7,014,942; 6,982,073; 6,958,196; 6,902,745; 6,844,099; 6,803,141; 6,803,138; 6,656,588; 6,589,680; 6,379,417; 6,165,553; 5,993,511; 5,958,361; 5,358,695; and 5,261,944. U.S. Pat. No. 6,613,300 is considered of particular relevance to the present invention, and relates to doped, pyrogenically prepared oxides of metals and/or non-metals. U.S. Pat. No. 7,211,236 concerns methods of producing metal oxides by flame spray pyrolysis for use in SOFCs. U.S. Pat. Nos. 7,014,942; 6,844,099 and 6,589,680 concern methods for preparation of an anode for a solid oxide fuel cell in which a plurality of zircon fibers are mixed with a yttria-stabilized zirconia (YSZ) powder, forming a fiber/powder mixture; the fiber/powder mixture is formed into a porous YSZ layer, calcined and impregnated with a metal-containing salt solution. U.S. Pat. No. 6,982,073 and concerns the use of nano-sized stabilized zirconia in electrolytes of solid-state fuel cells. U.S. Pat. No. 6,958,196 relates to porous electrodes comprised primarily of ceramic material and electronically conductive material for use in solid oxide fuel cells. U.S. Pat. No. 6,902,745 relates to a method for producing nano-sized lithium-cobalt oxide using flame-spray pyrolysis. The use of AR, H, O2/air, and air is disclosed. U.S. Pat. No. 6,803,141 describes ultra-high power density solid oxide fuel cells (SOFCs) which employs a buffer layer of doped-ceria, a zirconia electrolyte and a cobalt iron-based electrode. Micron thick layers are disclosed. U.S. Pat. No. 6,803,138 concerns processes for preparing aqueous suspensions of a nanoscale ceramic electrolyte material such as yttrium-stabilized zirconia. U.S. Pat. No. 6,656,588 discusses doped, nanosize metal oxide particles in a two oxide system in which a nano phosphorus powder and triethanolamine (TEA) are employed. U.S. Pat. Nos. 6,379,417; 5,993,511 and 5,261,944 concern nickel cermet anodic material for fuel cell anodes. U.S. Pat. No. 6,165,553 concerns methods of fabricating ceramic membranes. U.S. Pat. No. 5,958,361 relates to the use of flame spray pyrolysis to provide ultrafine metal oxide powders; glyoxylate polymetaloxane coatings are discussed. U.S. Pat. No. 5,358,695 discusses nanoscale ceramic particles.
SOFCs have utility in a wide range of power generation applications including residential, communications, commercial, industrial and military/security applications. SOFCs present a number of advantages compared to other types of fuel cells and existing power generation systems, such as (a) high-energy conversion efficiency (up to 40%-60%), (b) flexibility in fuel choice (natural gas, diesel, gasoline, liquid petroleum, biomass, etc.), (c) low levels of toxic emission (compared to conventional combustion electricity generation, carbon dioxide emissions are up to 60% lower and nitrogen oxides (NOx) and sulfuric oxides (SOx) can be minimized depending on the fuel input), (d) solid state device (SOFC technology has no moving parts or corrosive liquid electrolytes), (e) competitive production costs, and (f) broad product range capability. In contrast to PEMFCs, SOFCs are composed entirely of solid-state materials, typically ceramics. Since SOFCs do not employ water or membranes, they are capable of operating at much higher temperatures (e.g., 700-1,000° C.) than PEMFCs. The higher operating temperature of SOFC's eliminates the requirement for expensive catalysts, and permits SOFCs to be used with a variety of different fuels.
Typically, each fuel cell of an SOFC is composed of three layers: the cathode, the electrolyte and the anode. The cathode is the positive side of the cell (i.e., towards which electrons flow); its purpose is to use electrons to produce oxygen ions by reducing oxygen molecules in the air. As such, it must be an electrically conductive and air-porous material. The electrolyte is a dense, electrically insulating, gas-tight layer that separates the cathode from the anode, thereby requiring electrons resulting from the oxidation reaction on the anode side to travel through an external circuit before reaching the cathode side. The most important requirement of the electrolyte however is that it must be able to conduct oxygen ions from the cathode to the anode. It is common for the anode and cathode layers of such fuel cells to be connected in series using a metallic or ceramic layer (i.e., an anode “interconnect” and a cathode “interconnect”).
The anode is the negative side of the cell (i.e., from which electrons flow); its function is to use the oxygen ions that diffuse through the electrolyte to oxidize the hydrogen fuel, thereby producing water and electrical energy. The SOFC anode is used for the electrochemical oxidation of fuels such as hydrogen and natural gas. To minimize polarization losses in the H2 oxidation reaction, anode materials must exhibit high electronic conductivity, sufficient electrocatalytic activity for fuel oxidation reactions, chemically stability and thermal compatibility with other cell components and should have sufficient porosity for efficient gas transportation in high-temperature reducing environments.
In recent years, the Ni/YSZ (YSZ is Y2O3-stabilized ZrO2) cermet anode has been shown to be very promising for use in SOFCs. However, stability at high temperatures (˜1,000° C.) is required for long-term operation. In general, Ni grains in a Ni/YSZ cermet sinter easily at temperatures over 800° C., and this sintering leads to the degradation of the performance of the SOFC (Minh, N. Q. (1993) “Ceramic Fuel Cells,” J. Amer. Ceram. Soc. 76:563-588; Dees, D. W. et al. (1987) “Conductivity of Porous Ni/ZrO2—Y2O3 Cermets”, Solid State Ionics, 134:2141-2146). To obtain Ni/YSZ cermet anodes that are stable at high temperatures, it is essential to prevent sintering of Ni grains in the anode. It has been reported that an anode structure in which fine YSZ grains are dispersed on the surface of Ni grains improved the stability of a Ni/YSZ cermet anode (Fukui, T. et al. (1998) “Long-Term Stability of Ni-YSZ Anode With A New Microstructure Prepared From Composite Powder”, Electrochem. Solid State Lett. 1: 120-122). However, both high stability and high electrochemical activity are important for the Ni/YSZ cermet anode as the electrochemical activity strongly depends on a three-phase boundary (TPB) created among Ni grains, YSZ grains and pores. Thus, available anode materials are not fully satisfactory.
In sum, despite all prior advances, a need remains for improved SOFC anode materials. The present invention, which provides improved SOFC anode materials, and SOFCs which incorporate such anode materials, is directed to this and related needs.